A light-emitting diode (LED) is a semiconductor light source. An LED includes semiconducting materials doped with impurities to create a p-n junction, in which electrical current can easily flow one directionally from the p-side (anode) to the n-side (cathode), but not in the reverse direction. Charge-carriers (e.g., electrons and holes) flow into the p-n junction from connecting electrodes at each end of the junction having different voltages. For example, when an electron combines with a hole, the electron falls into a lower energy level and can release energy in the form of a photon, e.g., emitting light. This effect is referred to as electroluminescence. The wavelength of the light emitted, and thus the color of the emitted light, depends on the band gap energy of the materials forming the p-n junction. For example, bright blue LEDs are based on the wide band gap semiconductors including GaN (gallium nitride) and InGaN (indium gallium nitride). For producing white light using LEDs, one technique is to use individual LEDs that emit three primary colors (red, green, and blue) and then mix all the colors to form white light. Another technique is to use a phosphor material to convert monochromatic light from a blue or ultraviolet LED to broad-spectrum white light, e.g., in a similar manner to fluorescent light bulbs.
A laser diode (LD) is an electrically-pumped semiconductor laser light source. In an LD, the active medium is a solid state semiconductor formed by a p-n junction, e.g., similar to that found in an LED, rather than a gas medium (e.g., in conventional lasing). Laser diodes form a subset of semiconductor p-n junction diodes. For example, a forward electrical bias across the p-n junction of the LD causes the charge carriers to be injected from opposite sides of the p-n junction into the depletion or junction region, e.g., holes are injected from the p-doped component and electrons are injected from the n-doped component of the semiconductor material. As electrons are injected into the diode, the charge carriers combine, some of their excess energy is converted into photons, which interact with more incoming electrons, thereby producing more photons in a self-perpetuating analogous to the process of stimulated emission that occurs in a conventional, gas-based laser. Some examples of conventional LDs include 405 nm InGaN blue-violet laser diodes, e.g., used in in Blu-ray Disc and high definition DVD drive technologies, and 785 nm GaAlAs (gallium aluminum arsenide) laser diodes, e.g., used in Compact Disc (CD) drives.